A positive electrode active material for a lithium ion secondary cell has been generally constituted by a composite oxide of Li and a transition metal. In the composite oxide, lithium cobaltate (LiCoO2), which is a composite oxide containing Co as a component, has been frequently used. In recent years, there is increasing use of lithium nickelate (LiNiO2), lithium manganate (LiMn2O4), a ternary oxide (such as LiNi1/3Mn1/3Co1/3O2), and composite materials thereof.
As an electrolytic solution for a lithium ion secondary cell, solutions obtained by dissolving a lithium salt, such as LiPF6 and LiBF4 electrolyte, in a mixed solvent of a cyclic carbonate ester, such as PC (propylene carbonate) and EC (ethylene carbonate), and a linear ester, such as DMC (dimethyl carbonate), EMC (ethyl methyl carbonate) and DEC (diethyl carbonate), have been mainly used. These organic solvents are liable to be deteriorated in an oxidative atmosphere, and are particularly liable to undergo oxidative decomposition reaction on being in contact with a transition metal, such as Co, Ni and Mn, on the surface of the positive electrode. It is considered that the causes thereof include the high electric potential on the surface of the positive electrode, and the catalytic action of the transition metal in a highly oxidized state. Accordingly, for maintaining the capability of the electrolytic solution, it may be effective to prevent the electrolytic solution as much as possible from being in contact with the transition metal (such as one or more kinds of Co, Ni and Mn) constituting the positive electrode active material.